VulnerabilityPublished 20 Mar 2026

CVE-2026-33129: h3js h3 · Timing Side Channel

Affects h3js h3Source CVE Database V5

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Not known to be exploited · CVSS 5.9 Medium : upgrade to 2.0.1-rc.9 when convenient.

Data as of 5 Nov 2026, 00:01 UTC

CVSS 3.1 base score

5.9Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.3% chance of exploitation in 30 days – higher than 25% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.0.1-rc.9

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: None
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 20 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 5 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade h3js h3 to 2.0.1-rc.9.

Affected software

Product	Affected	Fixed in
h3jsh3	Affected >=2.0.1-beta.0 <2.0.1-rc.9	Fixed in 2.0.1-rc.9
GitHubh3js/h3`pkg:github/h3js/h3`Other threats	Affected =2.0.1-beta.0 =2.0.0-rc.8	Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

The primary mitigation is to upgrade all instances of the h3js framework to version 2.0.1-rc.9 or later, where the timing side-channel vulnerability has been fixed. For organizations unable to upgrade immediately, implement constant-time comparison functions in the authentication logic to prevent timing discrepancies. Additionally, introduce rate limiting and monitoring on authentication endpoints to detect and block repeated authentication attempts that may indicate timing attack probing. Employ network-level protections such as Web Application Firewalls (WAFs) configured to detect anomalous request patterns. Consider adding multi-factor authentication (MFA) to reduce the impact of compromised passwords. Finally, conduct code reviews and security testing to identify and remediate similar timing side-channel vulnerabilities in custom or third-party code.

Description

CVE-2026-33129 is a timing side-channel vulnerability in the h3js framework versions 2.0.1-beta.0 through 2.0.0-rc.8. The flaw exists in the requireBasicAuth function due to unsafe string comparison using ! ==, allowing attackers to measure response times and deduce valid password characters sequentially. This bypasses password complexity protections without requiring authentication or user interaction.

Impact

This timing side-channel vulnerability can lead to the compromise of authentication credentials by allowing attackers to deduce passwords character-by-character remotely. For organizations using vulnerable versions of h3js in their HTTP services, this means that attackers can bypass password complexity controls and gain unauthorized access to protected resources. The confidentiality of sensitive systems and data is at risk, potentially leading to data breaches or further lateral movement within networks. Although the vulnerability does not affect data integrity or availability directly, unauthorized access can enable attackers to perform malicious actions post-compromise. Since the flaw is remotely exploitable without authentication or user interaction, it increases the attack surface significantly, especially for internet-facing services. The medium CVSS score reflects that while exploitation requires some effort (high attack complexity), the impact on confidentiality is high. Organizations with critical services relying on h3js should consider this vulnerability a significant risk until patched.

CWE-208Observable Timing Discrepancy (opens in a new tab)
Base weakness
Two separate operations in a product require different amounts of time to complete, in a way that is observable to an actor and reveals security-relevant information about the state of the product, such as whether a particular operation was successful or not.
OWASP Cheat Sheets
Error Handling (opens in a new tab)
Logging (opens in a new tab)
OWASP ASVS 4.0.3
V8.3.4 (opens in a new tab)
V14.3.3 (opens in a new tab)
V7.3.3 (opens in a new tab)
Attack patterns
Cross-Domain Search Timing (opens in a new tab)
Application Fingerprinting (opens in a new tab)
System Footprinting (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 27 Mar 2026, 19:37 UTC

WeaknessesCWE-208

Technical details

References

Community reviews

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